US2836706A - Precision forging method and apparatus - Google Patents

Description

May 27, 1958 D. A. CAVANAGH 2,835,706

PRECISION FORGING METHOD AND APPARATUS Filed Nov. 22. 1955 3 Sheets-Sheet 1 FIGJ In ventor DA N/EL A. CA VANA 6H y 27, 1953 D. A. CAVANAGH 2,836,706

PRECISION FORGING METHOD AND APPARATUS Filed Nov. 22. 1955 3 Sheets-Sheet 2 Inventor DA N/EL A. CA VANAGH by! W May 27, 1958 CAVANAGH 2,836,706

PRECISION FORGING METHOD AND APPARATUS Filed Nov. 22. 1955 3 Sheets-Sheet 3 plastic yield pressure P in.

70' A temperature Inventor FIG. 8 DAN/EL A. CA VANAGH United States Patent PRECISION FORGING METHOD AND APPARATUS Daniel Alfred Cavanagh, Willowdale, Ontario, Canada Application November 22, 1955, Serial No. 548,483

9 Claims. (Cl. 219-150) This invention relates to methods and apparatus for forming precision forgings in one heating and forging operation at a precise forging temperature.

Conventional mechanical forging machines may be used to produce precision forgings by striking induction or flame heated bar stock into a precision die. A substantial temperature drop occurs during the transfer of the heated bar stock from the heating apparatus, whether of the induction, gas or resistance heating type, to the forging press which embodies the forging die. Allowance must be made for this characteristic in heating the bar stock to a temperature sufiiciently above the desired forging temperature to avoid undue cooling during transfer necessarily giving rise to grain growth. During stock transfer from the heating step to the forging operation, surface oxidation or scale forms to a degree limiting consistent reproduction to close tolerances and causing a high rate of wear in the forging die. Generally, therefore, precision forgings are accomplished by means of progressive forging operations employing a number of dies and a number of separate heating and forging operations except in cases where the amount of material being gathered and forged is very small. In many cases, the final forging operation comprises cold striking to attain finishing dimensions.

In any forging operation, temperature control is of utmost importance. In the case of a precision forging methods, a variation in forging temperature can cause variation in dimension beyond permissible tolerances. The 'heating step may be carefully controlled and yet during transfer to the forging operation, the bar stock may be subjected to unpredictable variables affecting the forging temperature to such an extent that the percentage of rejects is unduly high. The term precision forging as applied in the art heretofore merely signifies an attempt to forge to such tolerances that little or no machining is required thereafter though removal of die seam flash by shearing is generally accepted.

It is the main object of this invention to provide a method and apparatus for precision forging in which the forging temperature is precisely controlled at the moment of the forging operation and scale formation and the like is substantially obviated.

It is a further object of the invention to provide a method and apparatus for precision forging on production runs of large quantity with a low percentage of rejects due to causes other than temperature control.

It is a still further object of the invention to practice temperature control by means of a stock gathering operation under predetermined hydraulic or pneumatic pressure while a predetermined electrical current flows through the stock to develop a stock upset at a specific forging temperature and to immediately forge said upset to a precision shape at a specific and different forging pressure.

It is a still further object of the invention to provide a method and apparatus for heating bar stock or the like in a forgeable region thereof to a forging temperature and immediately upon cessation of said heating step, forging the forgeable region to a precision shape to form a substantially flashless forging.

It is a still further object of the invention to provide a method and apparatus for forging materials heretofore considered as non-forgeable and substantially any metal having a detectable plastic range and as short as twenty-five degrees Fahrenheit and shorter, such as, for example, cold working metals and alloys, difficult titanium alloys and high temperature metals and alloys of the recent metal arts.

It is a still further object of the invention to provide a method and apparatus for forming precision forgings in a die formed of conventional die material, wherein the bar stock or workpiece is heated by electrical resistance heating techniques.

With these and other objects in view, the invention will be appreciated by a study of the following specification taken in conjunction with the accompanying drawmgs.

In the drawings:

Figure 1 is a plan view of a preferred form of precision forging apparatus according to the invention;

Figure 2 is an elevation of the apparatus of Figure 1 embodying a schematic layout of hydraulic and electrical controls therefor;

Figure 3 is a perspective view of a precision forging which may be formed according to the invention in effectively one machine operation;

Figure 4 is a diagrammatic sectional view illustrating the method of the invention diagrammatically showing the location of the operative components at the beginning of a heating, stock gathering and forging cycle;

Figure 5 illustrates a material upset formed by gathering of bar stock according to the invention under a stock gathering pressure P at the moment of application of additional pressure P Figure 6 illustrates the precision forging of the upset formed by stock gathering as shown in Figure 5 into a die;

Figure 7 shows plastic yield pressure-temperature curves for a number of materials; and

Figure 8 is a transverse sectional view of a specific form of brush contacting mechanism of the invention disclosing electrical contact over an entire circumferential surface of the bar stock workpiece being forged.

Referring to the drawings, a machine bed 10 having an abutment block 11 slidably supports a forging die 12 movable axially by means of the cross bars 13 thereof under action of the pistons 14 of the parallel spaced apart hydraulic cylinders 15 fixed on the machine bed. The die 12 is separable and embodies the upper and lower parts or jaws 16 and 17 which may be firmly held together by means of suitable bolts 18 or other high pressure clamping means manually or hydraulically actuated in accordance with well known mechanical principles and methods. A length of bar stock 19 or the like rests in the lower part 17 of the die 12 and is held therein under suitable clamping pressure by means of the clamping bolts 18 described, pressing the upper part 16 of the die into firm engagement therewith. The die 12 embodies a precision cavity into which a portion of the bar stock 19 is to be gathered and forged. An electrical contacting brush 20 makes electrical contact with the bar stock 19.

The bar stock 19 initially projects beyond the die 12 for engagement of the contact end 21 thereof by an anvil block or head 22 supported by piston 23 of hydraulic pressure cylinder 24 supported by the machine bed 10. The anvil block 22 is connected to a source 25 of alter- Patented May 27, 1958 nating current as is the contact brush 20 from which it is insulated in any suitable way, to cause electrical resistance heating of that portion of the bar stock 19 referred to herein as the forgeable region 26 extending between the contacting brush 20 and the anvil 22. A suitable switch 27 is actuated to control the application of heating current to the foregeable region 26 The method of the invention is illustrated in Figures 4 to 6 wherein like numerals indicate like components. Referring also to Figure 2, the bar stock 19 is first supported in any suitable way between the anvil 22 and the abutment 11 under a predetermined axial hydraulic pressure P effecting electrical contact between contact surfaces 28 of the anvil and the contact end 21 of bar stock 19. A second electrical contact is made to the bar stock by means of the contact brush Zti shown in Figures 4 to 6 having a brush element 29 preferably formed of hard copper or the like and acting under pres sure of a spring 30 or other suitable means such as an hydraulic pressure device (not shown) and connected to source 25 and switch 27 substantially as shown. In Figure 4, the brush 29 is initially spaced a relatively short distance D from the contact end 21 of the bar stock and is insulated from head 22 by insulation 22a. It is preferred that the distance D be initially a minimum so that a fast heating of the forgeable region 26 will be accomplished by flow of electrical current from the source 25 through a minimum electrical resistance upon closure of switch 27.

According to the invention, the temperature at which stock gathering occurs is dependent upon hydraulic pressure P as hereinafter set forth in more detail. Thus, as heating of the forgeable region 26 increases, the anvil or head 22 will remain substantially stationary until the material of the forgeable region attains a temperature at which the material under pressure P yields and begins gathering. By this method and means, the stock gathern mp u n be con r l d in a p e s nn by con of th press re 1 to a pre erm ed value- The lower the pressure E the higher the stock gathering temperature. The pressure P is therefore thatcontrol ling the stock; gathering temperature to a value adapted for an immediately following forging step.

As the fcrgeable region 26 attains a temperature sufiicient to permit stock gathering, i. e., the particular temperature-yield pressure point in plastic temperature range has been achieved, the anvil head 22 advances under the ram pressure P for such particular temperature to, efiect an enlargement or upset 31. During the stock gathering operation, current flow is maintained through the'forgeable region of the bar stock. It will also be observed that as the stock gathering operation proceeds, the length of the forgeable region is substantially maintained by ad. vancing the brush 29 over the bar stock 19. The brush 20 is moved along the bar stock from the moment at which current flow begins to provide fresh and efficient electrical contact during the electrical heating step and to avoid undue heating of the brush contacting surfaces. The anvil head 28 does not move from the position illustrated in Figure 4 until the forgeable region has attained a temperature at which stock gathering can begin under action of the predetermined stock gathering pressure P The brush 29 may be supported by a separate carrier (not shown) or the die 12. As shown in Figures 1 and 2, the die is located between the brush contact and anvil 22. It is significant that during the stock gathering operation, the die precisely controls the contours of the neck portion 32 of the upset being formed.

As shown in Figure 2, the hydraulic fluid pumps 33 and 34 delivering hydraulic fluid under pressure from fluid sump 35 connect to the double two-way valve 36 by means of which the anvil head 22 is movable under the pressure P for stock gathering when the forgeable region attains a stock gathering temperature. The die 4 including the brush 29 is movable toward the abutment 11 under action of the hydraulic cylinders 15 at pressure P as modified by the valve 37 which may be of the restriction type whereby the rate of advance of the die and brush 2? may be controlled. By this means, the shape of the upset 31 formed in the stock gathering operation can be controlled to suit the form necessary for the subsequent forging operation to precision tolerance in the die cavity 33. The rate of advance of the die 12 as controlled by the valve 37 will determine the final spacings ranging between values S and S shown in Figure 5 between the anvil face 28 and the die at the conclusion of the stock gathering operation. The simultaneous opening of switch 27 and support of die 12 in a stationary position for support of the bar stock during the immediately following forging operation may be accomplished by means of a number of well-known mechanisms operating in accordance with this disclosure in the manner suggested in Figure 2. Thus, as the die 12 arrives at its limiting position of motion, in this case against the abutment 11, actuating mechanism 39 of any suitable form operatively connected to both switch 2'7 and high pressure valve 40 controlling the outlet of pump 34, is engaged by the die to cause switch 27 to open, thereby effecting cessation of electrical current flow through the bar stock and simultaneously opening valve 49 causing hydraulic pressure applied against the anvil head 22 to be increased by additional pressure P to a predetermined value.

The cessation of current fiow and preferably simultaneous increase of pressure, mark the beginning of the forging step or stroke of the anvil head 22 during which the gathered material represented by the upset 31 while at substantially ideal forging temperature, is forced into the die cavity 38 to form in one forging operation, the final precision forged product 41 shown also in Figure 3. The mechanism may be returned to the initial position illustrated in Figures 1 and 4 by turning valve 36 for reverse action of the hydraulic cylinders and since in the reversing action the anvil head 22 will be drawn away from the die 12, closure of switch 27 will be accomplished while the anvil head is removed from contact with the finished upset article 41. Moreover, upon closure of switch 27, the valve 40 is closed whereby the mechanism is returned to the initial position under action substantially of the pump 33 alone.

It must be borne in mind, that in any stool; gathering operation according to the invention, it is desired to attain a temperature which is suitable for an immediately following forging operation. Since the stocl; gathering op eration is accomplished under hydraulic pressure, the correct hydraulic pressure to be applied will be that corresponding'to the pressure necessary to deform the material at the deforming or stock gathering temperature. The pressure necessary to cause plastic flow will depend upon the temperature in the manner indicated by the trend curves shown in Figure 7 for aluminum, copper, steel and stainless steel. Experiments by applicant on a number of materials have shown that precise curves can be developed for a given material and will be somewhat of the form illustrated.

For example, assuming it is desired to accomplish the die forging step at 2,000 F, then the initial stocl; gathering pressure should be sutficiently low that stock gathering will not begin until the forgeable region has attained a temperature of 2,000" F. If this initial pressure were greater, stock gathering would begin at a lower temperature and then the formed upset would not be at a sufficiently high temperature to undergo successful precision forging into. the die cavity in the latter step of the method. It will be evident, therefore, that the stock gathering pressure is used according to this invention as a temperature control determining both the stock gathering temperature and the temperature at which the following die forging operation proceeds. One first determines the forging temperature required from which the pressure P can be determined.

It has been found impractical to maintain electrical current flow through the bar stock in the last die forging step of the method herein in the case of the formation of a part such as the die forging 41 having a slightly bulged finished edge 42 because the anvil face 28 and the die arrive in such close proximity that electrical current flow therebetween through a small portion of the workpiece or bar stock therebetween may cause excessive heating or other damage to the anvil face 28 or the die or both, as well as to the workpiece or forging 41. The invention permits the use of ordinary die materials for the forging die and ordinary materials for the anvil face rendering impractical the use of high cost high resistance low thermal conductivity die materials made necessary if current flow is maintained during forging stroke. Therefore, the severance of electrical current flow before the completion of the die forging operation is preferred and will be found to be highly practical in nearly all cases.

While a single contact brush has been disclosed for purposes of convenience in Figures 1 to 6, the use of a single brush contact may be impractical due to nonuniform current flow in many precision forging operations. It is therefore preferred to provide contact about an entire circumferential area of the workpiece as shown in the case of workpiece 43 shown in Figure 8. Copper brushes 44 supported by holders 45 slidable radially and connected by flexible leads 46 to electrode 47 of the electrical current supply system (not shown). Holders 45 are actuated by hydraulic cylinders 48 mounted on carrier plate 49 to effect contact of brushes 44 under substantial pressure with workpiece 43. The brushes are moved along the workpiece by movement of the carrier plate 49 and the assembly shown with the forging die or independently. The brushes are separated by an angle of one hundred and twenty degrees in the arrangement shown. Applicant has established that more than two brushes should be used to provide efficient circumferential contact.

Precision forgings have been formed according to the invention to tolerances within .002 inch maintained over a production run with a material having a plastic range as short as F. in a forging die formed of conventional die steel. It will be apparent that the apparatus shown may be fed with bar stock by any of the well known automatic feeding systems and that likewise the finished parts may be discharged by the practice of well known mechanical expedients. Moreover, a specific form of pressure and electrical control may be provided in a degree of automation desired according to well known principles and techniques and with conventional apparatus without departing from the scope of this invention.

A higher pressure is required for precision forging as compared with mere stock gathering. If stock gathering is accomplished at a forging pressure, the temperature will be low for a precision forging operation under an hydraulic pressure system as above outlined. If the material is heated beyond a forging temperature, then it can be puddled into an insulated mold cavity while current flow is maintained but enlarged grain structure of the metal results and the low pressures available under such a method do not permit the duplication of parts in such a mold to satisfactory production tolerances which can be maintained. By way of contrast, the invention provides a method and apparatus in which a forging die rather than a mold is located about the stock intermediate or between the ends of the heated or forgeable region and between the electrical connections thereto whereby a precision workpiece component or part is formed under ideal forging conditions to deliver a consistency in form, dimension and metal grain structure heretofore unattainable by methods in which a heated part is transferred 6 to or otherwise brought into contact with a die previously separated therefrom.

This specification has set forth the requirement that the stock when gathered and ready for forging, be immediately forged. The meaning of the term immediately is best defined by giving consideration to a phenomenon investigated by the applicant and which appears to have a fundamental bearing on forging techniques. It has been determined by applicant that heat is dissipated from a heated part in such manner and at such a rate that the forging characteristics appear to change in a way which can be visualized by assuming for convenience the penetration of a cold wave into the metal. Thus, if a one inch square surface of a steel part at 2,000 F. is contacted with a one inch square surface of a steel body at room temperature, then the heat dissipates into the cold body at a rate represented by a rate of temperature drop penetration into the hot body of one inch in six seconds. The amount of temperature drop represented by the rapidly penetrating cold wave is dependent upon the heat absorption characteristics of the body contacting the heated steel part as well as the radiation and conductivity characteristics of the latter. Thus, air surrounding the heated steel part will still set up a cold wave travelling at the same rate but the temperature drop represented by the cold wave will be much less than the temperature drop were the part contacted by cold steel.

A very small temperature drop in the body to be forged can make a very great difference in the forging characteristics particularly if the material forged has a short plastic temperature range. A number of experiments conducted by applicants have shown that cold wave penetration of a workpiece ready for forging should not be greater than twenty-five percent of the minimum thickness of the part measured in the final forged state. For example, consider an automobile valve; the minimum thickness of the forged valve head would govern the maximum time which can be permitted between the heating and forging steps as well as the maximum time that can be permitted from completion of the heating step for complete precision forging operation.

The applicant has established that in general, the total forging time from the cessation of heating until'the completion of the forging operation, should be less than where F; is total forging time from cessation of heating to completion of forging; t is minimum thickness in the final forged parts; 6 is the cold wave penetration factor.

Table 1 Values for n for 25% cold wave penetration:

Steel 2.5 Brass 1.3 Copper 0.3 Aluminum 0.53 Nichrome 12.9

A cold wave penetration factor for penetration is the time required for one inch of penetration of a cold wave into the workpiece.

It will be apparent from the foregoing that where the forged part is of very great thickness at a minimum section, say of two to three inches, and/or the plastic temperature range is very long, then a number of seconds #7 is short, prior forging practice has not satisfactorily solved the delivery of grain structure uniformity and maintenance of dimensions throughout a production run of a large number of pieces and metals, of less than about 200 F. plastic temperature range have been inoperative for forging techniques.

Accordingly, the phrase immediately forging at forging temperature as used in this specification, refers to completing the forging operation before the cold wave penetration exceeds a predetermined value. In this connection, the degree of cold Wave penetration permissible, has been arbitrarily set as at a figure of twenty-five percent representing good practice with most materials. However, a material having a very short range of plasticity, may only be capable of delivering a forging of uniformly satisfactory grain structure if the cold wave penetration is kept to less than say ten percent. With other materials, the permissible cold wave penetration may be as high as say forty percent. Also, a greater cold wave penetration may be permissible on contact with air because the temperature drop represented by the cold wave may be small and this will be the case in particular with materials having a long plastic temperature range.

Recognition of the time for cold wave penetration as set forth in this specification has enabled the successful forging of materials having a plastic range as short as 10 F. and it appears practical Within the limits of this specification to successfully forge materials of a plastic range as short as 5 or less, subject to mechanical limitations of forging mechanism and radiation effects and heat absorption effects of the forging surfaces of the die and anvil.

While there has been described what is at present considered a preferred embodiment of the present invention, it will be appreciated by those skilled in the art that various changes and modifications can be made therein without departing from the essence of the invention and it is intended to cover herein all such changes and modifications as come within the true spirit and scope of the appended claims.

What I claim as my invention is:

1. The method of forging a workpiece comprising the steps in combination of: applying pressure to said workpiece; heating said workpiece while under pressure; controlling said pressure to a value insufiicient to cause said workpiece to deform thereunder until the workpiece attains a desired forging temperature, thereby to control the temperature at which the deformation of the work piece occurs; and immediately forging said workpiece into a die at said forging temperature.

2. The method of forging a workpiece comprising the steps in combination of: applying pressure to said workpiece; heating said workpiece while under pressure; controlling said pressure to a value insufficient to cause said workpiece to deform thereunder until the workpiece attains a desired forging temperature, thereby to control the temperature at which deformation of he workpiece occurs; and forging said workpiece into a die in one forging operation at said forging temperature within a predetermined cold wave penetration time.

3. The method of forging a workpiece comprising the steps in combination of: applying pressure to said workpiece; heating said workpiece while under pressure; controlling said pressure to a value insufiicient to cause said workpiece to deform thereunder until the workpiece attains a desired, forging temperature, thereby to control the temperature at which deformation of the workpiece occurs; discontinuing heating of said workpiece; and forging said workpiece into a die in one forging opera tionwithin a period of time from the moment of discontinuance of said heating operation less than that required for a predetermined cold wave penetration into said workpiece.

4. The method of forging a workpiece to a precision finished shape comprising the steps in combination of: applying a pressure to said workpiece; electrically contacting said workpiece at spaced apart areas thereof to effect heating of a heatable region therebetween by flow of electrical current therethrough while making at least one of said contacts about an entire circumferential area of said workpiece; controlling said pressure to a value insuificient to cause said workpiece to deform until the latter attains a forging temperature determined thereby and at which gathering of the material of said workpiece occurs under said pressure; and forging the gathered material of said workpiece while at said forging temperature into a forging die under a pressure greater than said material gathering pressure.

5. The method of forging a workpiece to a precision finished shape comprising the steps in combination of: applying a pressure to said workpiece; electrically contacting said workpiece at spaced apart areas thereof to effect heating of a heatable region therebetween by flow of electrical current therethrough while making at least one of said contacts about an entire circumferential area of said workpiece; controlling said pressure to a value insufficient to cause said workpiece to deform until the latter attains a forking temperature determined thereby and at which gathering of the material of said workpiece occurs under said pressure; forging the gathered material of said workpiece while at said forging temperature into a forging die under a pressure greater than said material gathering pressure; and immediately before forging said gathered material, severing electrical current flow through said workpiece.

6. The method of forging a workpiece to a precision finished shape comprising the steps in combination of: applying a pressure to said workpiece; electrically contacting said workpiece at spaced apart areas thereof to effect heating of a heatable region therebetween by flow of electrical current therethrough while making at least one of said contacts about an entire circumferential area of said workpiece; controlling said pressure to a value insufficient to cause said workpiece to deform until the latter attains a forging temperature determined thereby and at which gathering of the material of said workpiece occurs under said pressure; moving the circumferentially contacting area in a direction along the workpiece effecting continuous contact with unheated material of said workpiece; and forging the gathered material of said workpiece while at said forging temperature into a forging die under a pressure greater than said material gathering pressure.

7. Precision fonging apparatus adapted to forge a workpiece in one heating and forging cycle comprising in combination: means for applying pressure to said workpiece; means for passing an electrical current through at least a portion of said workpiece while under pressure; a forging die having a die cavity extending about said workpiece adjacent the heated portion thereof; means including a pressure source for controlling said pressure to a value insufficient to cause said workpiece to deform. until the latter attains a forging temperature determined thereby and at which the heated material of said workpiece is gathered under said pressure; and means for applying an increased press to the gathered material of said workpiece whiie at forging temperature to force said material into said die to form a forged shape on said workpiece defined by the cavity of said die.

8. Precision forging apparatus adapted to forge a workpiece in one heating and forging cycle comprising in combination: means for applying pressure to said workpiece; means for passing an eiectrical current through at least a portion of said workpiece while under pressure; a forging die having a die cavity extending about said workpiece adjacent the heated portion thereof; means controlling said pressure to a value insufficient to cause said workpiece to deform until the latter attains a forgsasasgroe ing temperature determined thereby and at which the heated material of said workpiece is gathered under said pressure; means for applying a greater pressure to the gathered material of said workpiece while at forging temperature to force said material into said die to form a forged shape on said workpiece defined by the cavity of said die; and means for severing electrical current flow through said workpiece operative substantially upon application of said greater pressure applying means.

9. Forging apparatus comprising in combination: means for applying axial pressure to a workpiece; means including a source of electrical current for contacting one end of said workpiece to define one contact area and for contacting substantially a circumferential area of said workpiece between the ends thereof to define another contact area and providing electrical resistance heating of a. portion of said workpiece between said contact areas while under pressure; a forging die having a die cavity adapted to extend about said workpiece adjacent the heated portion thereof while under pressure and between the contact areas thereof; means for controlling said pressure to a value insuflicient to cause the i0 heated portion of said workpiece to deform until the latter attains a forging temperature determined thereby to accomplish both precise control of said temperature and stock gathering of the heated material of said workpiece under said pressure; and means for forging the gathered stock of said workpiece while at forging temperature by a forging pressure greater than said controlled stock gathering pressure into said die cavity in one forging stroke.

References Cited in the file of this patent UNITED STATES PATENTS 396,010 Thomson Jan. 8, 1889 437,654 Lauder et al Sept. 30, 1890 1,174,446 Rietzel Mar. 7, 1916 1,259,275 Murray Mar. 12, 1918 l,772,444 Giacchino Aug. 5, 1930 1,870,987 Drake Aug. 9, 1932 2,016,728 Sciaky Oct. 8, 1935 4 2,405,033 Grimes July 30, 1946 29 2,667,558 Aeckersberg 61; al Jan. 26, 1954

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